Fluoride ion affinity

Comment on heats of formation of fluoro-anions, and electron and fluoride-ion affinities of neutral fluorides, measured mass spectropho-tometrically (57,185,216,222) or derived from salt values obained by conventional calorimetry (32, 45, 46, 105) needs to be reserved until better agreement is reached between methods. However, from measurements on heats of formation of the predominantly ionic xenon fluoride adducts it has been possible to show the trend to increasing ionic-ity with pentafluoride partners Nb < Ta < Sb, which parallels the increasing Lewis acidity of these fluorides found by independent methods (44). 

More recently, a quantitative scale for Lewis acidity based on fluoride ion affinities was calculated using ab initio calculations at the MP2/B2 level of theory.26 Due to its high basicity and small size, the fluoride ion reacts essentially with all Lewis acids thus the fluoride affinity (or reaction enthalpy) may be considered as a good measure for the strength of a Lewis acid. An abbreviated pF scale is given in Table 1.3. This scale was used recently by Christe and Dixon112 for estimating the stability of salts of complex fluoro anions and cations. The pF value represents the fluoride affinity in kcal mol 1 divided by 10. 

Several intermediate antimony halides, such as (SbF3)I(SbF5)y, with jc = y — 1 x = 6 and y = 5 x = 2 and y = 1 x = 3 and y = 1 are known. Their existence, and their structures, depend on the high fluoride ion affinity of SbF5, whereby SbF6 and Sb2F i ions and complex polymeric cations, such as (Sb3Fg ) are formed. 

An alternative view of the interaction of an alkali metal cation with a fluoride-containing anion is one of Lewis acid/base competition. The reactions discussed in the preceding section involved the reaction of an alkali fluoride salt with a Lewis acid with subsequent fluoride ion transfer to the Lewis acid. However, the alkali metal cation is a Lewis acid as well, and the degree of perturbation of the anion by the cation may be dependent on the differences in fluoride ion affinity of the Lewis acid and the alkali metal cation. The fluoride ion affinities for a variety of Lewis acids are well known from ICR (53,54,64) studies, while the fluoride ion affinities for alkali metal cations are the heterolytic bond dissociation energies of the gas phase alkali fluoride molecules... 

In their study of the fluoride ion affinities of various fluoroacids Mallouk et al. (Mallouk, T. E. Rosenthal, G. L. Muller, G. Brusasco, R. Bartlett, N. Inorg. Chem. 1984, 23, 3167-3173) employed the linear dependence of lattice energy upon the inverse of the cube root of the formula unit volume to determine the lattice energies of salts of unknown structure. From that empirical relationship which is (kcal mol" ) = 556.3 (molar volume in A )- / + 26.3, l/UCfiFfiAsPs) =115 kcal mol" and C/L(C,oFgAsF6) 108 kcal mol". ... 

For the Madelung part of the lattice energy, in the case of a salt for which the structure is accurately known, we employed the method of E. F. Bertaut as modified by D. H. Templeton, and evaluated other terms as set out in Ref. 82. When applied to salts that dissociate easily into gaseous molecules, this provided us, via the vant Hofi relationship (see Refs. 80, 82, and 105) with evaluation of the enthalpies of ionization, AH gg [EFa (g) (EFi i)+(g) -I- F (g)] for a variety of F donor molecules, and the fluoride-ion affinities for well known F acceptors, e.g. Ap(BF3) = — AH29g [BF3(g) + F (g) BF4 (g)]. Such evaluations help to provide a more quantitative evaluation of possible reaction chemistry. The intercalation of graphite by one-electron oxidizers and by fluoride-ion acceptors, as covered in Chap. 10, provides examples. Such evaluations are also useful in assessing the likelihood of the existence of salts not yet known, such as [ArFJ fMFg] (see Ref. 92). 

The discovery of what appears to be a thermodynamic threshold governing the intercalation of graphite by fluoro anions, MF,, has required the evaluation of the thermodynamic stability of a number of such species. Since germanium tetrafluoride and fluorine are intercalated, in combination, by graphite to form both GeFj" and GeFs ", the first and second fluoride ion affinities of that molecule are each of interest. Evaluation of the fluoride ion affinity of boron trifluoride by Altshuller yielded a value of-71 kcal moF. This has been accepted by several authors as the basis for other fluoride ion affinities and electron affinities. Sharpe, however, has preferred a value of -91 kcal moF, based upon the data of Bills and Cotton. Although this latter value is in harmony with other fluoride ion affinities and electron affinities, its confirmation was clearly desirable to provide a firm basis for correction of affinities based upon the lower value. This paper describes the studies that have provided these fluoride ion affinities. 

Fluoride Ion Affinities of GeF4 and BF3 Table vn. Lattice Enthalpies and Basic Radii ... 

This is gratifying close to the value of-l33 kcal mol" (see Table IX) derived in this study using the salt NOUFj. This agreement supports both the correctness of the value for the fluoride ion affinity of BFj and the lattice energy evaluations. 

For the determination of the double fluoride ion affinity of Gep4, using the readily dissociated salt (Sp3)2GeF4, the fluoride ion donor enthalpy of SF4 had to be found. This was achieved via the salt SF3BF4, as revealed in Table X. This provides... 

The derivation of the double fluoride ion affinity of GeF4 is given in Table X. The relevant enthalpy change is... 

The fluoride ion affinity of 111 kcal mol l given previously was conservatively evaluated. The value is probably higher and could be as great as 115 kcal mopl. Berkowitz, W.A. Chupka, P.M. Guyon, J.H. Holloway, and R. Spohr, J. Phys. Chem., 1971, 75, 1461. 

Beauchamp and Murphy [12] have concluded that the fluoride-ion affinity of SiF4 is only slightly inferior to that of BF3. Since the SiFs", although roughly 5/4 times the volume of BF4, requires no additional separation of the carbon-atom sheets [13] than does BF4, the energy expended for salts of comparable charge, Cy EF,c , should be similar. It seemed therefore that graphite salts Cy SiFs should exist [14]. 

T. E. Mallouk, G. L. Rosenthal, G. Muller, R. Brusasco and N. Bartlett, Fluoride Ion Affinities of GeF4 and BF3 from Thermodynamic and Structural Data for (SF3)2GeFe, GlOaGeFs, and CIO2BF4, J. Inorg. Chem. 23 (1984) 3167-3173. 

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